# std::basic_filebuf<CharT,Traits>::open

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< cpp‎ | io‎ | basic filebuf

basic_filebuf* open( const char* s, std::ios_base::openmode mode ); | (1) |
basic_filebuf* open( const std::string& str, std::ios_base::openmode mode ); | (2) | (since C++11)
basic_filebuf* open( const std::filesystem::path& p, std::ios_base::openmode mode ); | (3) | (since C++17)
basic_filebuf* open( const std::filesystem::path::value_type* s, std::ios_base::openmode mode ); | (4) | (since C++17)

If the associated file was already open (is_open() != false), returns a null pointer right away.

Otherwise, opens the file with the given name (s, p.c_str()(since C++17) or str.c_str(), depending on the overload). std::ios_base::openmode values may be written as, e.g., std::ios_base::out | std::ios_base::app.

Overload (4) is only provided if `std::filesystem::path::value_type` is not char. | (since C++17)

The file is opened as if by calling std::fopen with the second argument (file access mode) determined by the result of mode & ~std::ios_base::ate as follows, `open()` fails if the result is not some combination of flags shown in the table:

mode & ~std::ios_base::ate | ﻿std::fopen﻿ access mode | Action if file already exists | Action if file does not exist
--- | --- | --- | ---
`binary` | `in` | `out` | `trunc` | `app` | `noreplace` (since C++23)
- | + | - | - | - | - | "r" | Read from start | Failure to open
+ | + | - | - | - | - | "rb"
- | + | + | - | - | - | "r+" | Error
+ | + | + | - | - | - | "r+b"
- | - | + | - | - | - | "w" | Destroy contents | Create new
- | - | + | + | - | -
+ | - | + | - | - | - | "wb"
+ | - | + | + | - | -
- | + | + | + | - | - | "w+"
+ | + | + | + | - | - | "w+b"
- | - | + | - | - | + | "wx" | Failure to open | Create new
- | - | + | + | - | +
+ | - | + | - | - | + | "wbx"
+ | - | + | + | - | +
- | + | + | + | - | + | "w+x"
+ | + | + | + | - | + | "w+bx"
- | - | + | - | + | - | "a" | Write to end | Create new
- | - | - | - | + | -
+ | - | + | - | + | - | "ab"
+ | - | - | - | + | -
- | + | + | - | + | - | "a+"
- | + | - | - | + | -
+ | + | + | - | + | - | "a+b"
+ | + | - | - | + | -

If the open operation succeeds and (openmode & std::ios_base::ate) != 0 (the `ate` bit is set), repositions the file position to the end of file, as if by calling std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling std::fopen. If the repositioning fails, calls close() and returns a null pointer to indicate failure.

### Parameters

s, str, p | - | the file name to open; s must point to a null-terminated string
openmode | - | the file opening mode, a binary OR of the std::ios_base::openmode modes

### Return value

this on success, a null pointer on failure.

### Notes

`open()` is typically called through the constructor or the `open()` member function of std::basic_fstream.

### Example

Run this code

    #include <fstream>
    #include <iostream>

    int main()
    {
        std::string filename = "Test.b";
        std::filebuf fb;

        // prepare a file to read
        double d = 3.14;
        if (!fb.open(filename, std::ios::binary | std::ios::out))
        {
            std::cout << "Open file " << filename << " for write failed\n";
            return 1;
        }
        fb.sputn(reinterpret_cast<char*>(&d), sizeof d);
        fb.close();

        // open file for reading
        double d2 = 0.0;
        if (!fb.open(filename, std::ios::binary | std::ios::in))
        {
            std::cout << "Open file " << filename << " for read failed\n";
            return 1;
        }

        auto got = fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);
        if (sizeof(d2) != got)
            std::cout << "Read of " << filename << " failed\n";
        else
            std::cout << "Read back from file: " << d2 << '\n';
    }

Output:

    Read back from file: 3.14

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 596 | C++98 | `open()` could not open files in append mode | can open in append mode

### See also

is_open | checks if the associated file is open (public member function)
close | flushes the put area buffer and closes the associated file (public member function)

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